The romantic in me so wanted to believe Ed Miliband when, in announcing his marriage to his partner, Justine Thornton, he said in that slightly clumsy way that even otherwise uber-articulate men do, when talking about such things: "I feel incredibly privileged to be marrying someone so beautiful and who is such a special person." I could feel the "aargh" forming on my lips.

The British public's appetite for someone else's wedding appears undiminished, even if the numbers actually tying the knot have been in step decline to a record low in 2008. With Miliband's announcement, a cash-strapped nation can now look forward to not one but three high-profile big days: Prince William and Kate's on 29 April, Ed and Justine's on 27 May and Zara Phillips and Mike Tindall's on 30 July. But then the Labour leader slightly took the edge off it. "At the end of the day we're in our 40s and we've got two kids, so it wasn't a case of me suddenly popping the question," he added in his exclusive interview with his constituency paper, the Doncaster Free Press.

Suddenly, he was making it sound more like an exercise in tying up a few loose domestic ends. Which, of course, is a very modern view of what was once the sacrament of marriage.

Then, of course, there is the inevitable suspicion that the small family ceremony the couple is planning for environmental lawyer Thornton's hometown of Nottingham is simply a move to head off criticism from the Daily Mail, which likes to refer to Miliband as "the first leader of a major political party to live with his family out of wedlock". It doesn't quite use the phrase "living in sin" – or "over the brush", as my Auntie Rita still calls it – but the inference is still clear. You can't be prime minister material if you don't marry your partner. That equation of personal moral code and public action apparently still has a resonance in Middle England.

As ever, there is an edge of hypocrisy in all this. While Miliband is criticised for not marrying the mother of his two young sons, Cherie Blair is savaged when she presents a picture of such conjugal bliss with Miliband's predecessor but one. If we really required a happily married, faithful family man at the national helm, then her remark that Tony "still excites me in every possible way" should have had us purring with delight that all is well with the world.

But there is a serious point here. In defending his decision hitherto not to get spliced to Thornton, Miliband has also declared marriage "a very important institution", albeit one that was less important for him than going to the Copenhagen Climate Change Summit, a general election and a Labour leadership election where he defeated his older brother, David (who will not be best man, something Labour's spin doctors are trying lamely to present as typical of Ed's modernity). So if he says that marriage is important, then he should act on it. And here Miliband is being as good as his words.

The question, though, is: important to whom? Why does marriage matter anymore? Or does it?

Miliband's previous comments on the subject – including a rather edgy, defensive exchange with Piers Morgan back in January, when the former newspaper editor turned TV star picked him up repeatedly for referring to Thornton as his "wife" – are better on to whom it doesn't matter than to whom it does. So, when asked if a prime minister needs to be married, he replied: "I think people are pretty relaxed about this."

That would seem to be a pretty sound judgement on the national mood regarding marriage, were it not for one thing. The prime minister continues to play a role, even if much-reduced, in the appointment of the bishops of the Church of England. And the Church of England teaches that the correct moral context for a sexual relationship is within marriage.

So, should someone who might be judged by his actions to be rather casual about marriage himself be involved in naming bishops, whose brief is to uphold the importance of the sacrament? In theory, it does make the system look foolish. Yet the reality is that even prelates (save the Pope) tend to take a rather relaxed line nowadays on the correct order for sex and marriage. Combined marriage and baptism ceremonies are increasingly popular after Church of England guidelines were relaxed in 2009 by its Liturgical Commission to accommodate them.

To many Anglicans, it was a sign that their church recognises and embraces the modern world – where 44 per cent of children are now born to unmarried mothers – but some traditionalists still objected to what they saw as a debasing of the core moral message. "It is a pity they haven't put in a funeral for grandma as well," complained Bishop John Broadhurst (who has subsequently departed for the Catholic Church).

It is that whole question of the establishment of the Church of England that still makes the whole area of marriage and moral conduct in their personal life a potentially tricky area for any aspiring prime minister. As things stand, it remains unthinkable, for instance, that Prince William and Kate Middleton could delay their big day until they have produced an heir and a spare at their love nest on Anglesey. Prince William will one day become the Supreme Head of the Church of England, and that requires that he marries at the altar and doesn't double-up with a christening in Westminster Abbey later.

But, for Miliband, an avowed atheist, such considerations surely no longer apply, even if one day he might be prime minister. Times have moved on, for example, since Neil Kinnock was given a hard time in 1988 after appearing on Wogan and saying he didn't believe in God. It may still be beyond the pale for American presidents to dismiss the Almighty (even if theirs is a country where church and state are formally separated) but in Britain in 2011, establishment or not, the question of a prime minister's religious beliefs is increasingly irrelevant. In highlighting the importance of marriage, Miliband has previously entered one vital caveat. It is not, he has said, "a pre-requisite for stable family life". Here he is chasing down David Cameron's flagship pre-election policy of giving tax breaks to married couples in an effort to tackle "broken Britain". Though the practicalities of Coalition Government have seen much of this pro-marriage Tory rhetoric remain as simply fine words, there is still a key delineation here.

There are many in Tory ranks who argue that there is a link between falling rates of marriage and children's educational under-achievement, delinquent behaviour and involvement with the criminal justice system. They highlight soundly based research that a child with two parents tends, statistically, to do better than the child of a single parent. But what the research doesn't prove, as Labour has pointed out, is that the two parents have to be married. It is their commitment to their child's future which contributes to building an "unbroken" society, not a ring on their fingers and a big white dress tucked away in plastic in the back-bedroom wardrobe.

The respective merits of cohabiting as against marriage are the subject of endless and endlessly contradictory research. Some surveys show that cohabiting couples are more likely to break up than similar couples, but "break up" can sometimes cover the pair deciding to get married. In these terms, Miliband and Thornton would count on 27 May as having broken up. And there is certainly no compelling evidence that the estimated 20 per cent of children born to cohabiting couples in the UK are markedly more likely to offend than those born to married parents.

So if marriage itself is not obviously beneficial to the children of the couple, where does that leave Miliband's description of marriage as "important"? Well, it can only mean that it is simply important to the couple themselves. As he has said: "Politicians should be able to make their own decisions about their lives."

There are many reasons why politicians and the rest of us choose marriage over cohabitation. One obvious reason is religion. Some churches like to characterise this as making the union between bride and groom not a question of joining two, but rather of three, because God is included. It is a line that has always made me feel slightly uncomfortable, with echoes of when Diana, Princess of Wales, remarked in her Panorama interview: "there were three of us in this marriage".

The real appeal of marriage in church, for me at least, is that its signifies that wedlock is something other than a legal and financial commitment. It has a spiritual dimension, and therefore is a sacrament, one of just seven, each marking the rites of passage. If we want a religious connotation to birth and death, in the sacraments of baptism and extreme unction (now usually called, somewhat lamely, the sacrament of the sick), why not in choosing a life partner, too? For that is what couples think they are doing when they walk up the aisle, and what they mean when they say their vows. No one in my experience keeps their fingers crossed as they say "I do". It is just that reality of life, in this as in other matters, can get in the way of the ideals that religion holds up.

But with church-going in steep decline, that religious dimension to marriage is increasingly less significant. That leaves only its legal and public dimensions. The first remains, if anything, as important as ever. With 50 on the horizon, I am at that stage in life where, one after the other, friends who are long-term cohabitees are deciding to marry so as to clear up any questions of rights to inherit each other's property and pensions. Most chafe at having to do it, and a small number protest that they would like to have the option of civil partnerships for heterosexual couples too, but finally they yield to the pragmatism of marriage.

And what of the public commitment – making a commitment before your peers? There may only be 50 guests at Langar Hall Hotel to see Miliband and Thornton betrothed, but it will have a public resonance. That is what standing up on the altar, or in front of the registrar, is all about: stating your intentions before your peers.

Some, though, don't see it that way. Gyles Brandreth, broadcaster, actor, sometime politician and fabled wearer of brightly coloured jumpers, talked recently on Desert Island Discs of how he and his wife of 42 years, Michele, waited until the birth of their first child to tell their parents they had already married in secret. "It was a private thing," he said. "If you are getting married, it's about the person you are marrying, it's about the relationship. For me it is not about the party." He also revealed it was something of a family tradition. His parents and some of his children had taken the same route.

Yet, if marriage retains its popular appeal in these secular, sceptical times, it is often precisely for the party. The relaxation under New Labour in rules about where weddings can take place has contributed hugely to this. Registry offices were so municipal, but now for those hankering for a venue their friends will never forget, the sky's the limit – literally. You can get married mid-air, or in the Palace of Westminster, a lighthouse, a zoo or at Disney World. Anywhere that could do with the booking fee.

And if it is the exotic you crave, you could follow the example of Darren McWalters and Katie Hodgson from Wales, who were strapped, standing up in their wedding gear, to the wings of two separate biplanes, while the (game) local vicar did a wing walk on a third, which flew in formation in front of the other two while vows were exchanged by megaphone. Or Sandra Eens and Jeroen Kippers, who plighted their troth on a platform that rose to 164ft in the air before the happy couple jumped off, attached to bungee ropes.

This, presumably, is not what Cameron has in mind with his talk of beefing up marriage. Spectacular weddings may tempt other couples to opt for their very own big adventure in the limelight, but such showpieces, for all their fanfare and photo opportunities, bear little correlation to the future stability of the marriage.

Indeed, the opposite may be true. The bigger the splash, the more likely the let down afterwards. Think 1981 and Prince Charles and Diana, in her fairy princess taffeta-and-lace dress by David and Elizabeth Emanuel. Perhaps Prince William and Ms Middleton might do better, even at this late stage, to opt for the £999 all-in package (dress, cake, ceremony, banquet for 30 guests and bridal suite) at the motorway motel that my 11-year-old daughter spotted (excitedly) recently.

Prince Charles and Diana's big day did, it was reported, caused a brief rally in the number choosing traditional church weddings, but it was as shortlived as their marriage. Which brings me back to Miliband's reluctance to cast his own nuptials in any wider context. External factors are rarely a good reason for getting married. We can tinker all we like with the legalities of marriage, the tax breaks it brings, even its religious significance, but if the two people concerned aren't essentially doing it for themselves and each other, then it is ill-fated. And rather than the death of romance or the decline of religion, it is the external pressures that have arguably done more damage to the institution of marriage than anything else.

Peter Stanford's book, 'The Extra Mile: A 21st Century Pilgrimage' is out in paperback this week from Continuum